Both densely burned, unglazed and glazed stoneware tiles are known from the literature. Both types of tiles have their advantages and disadvantages. Densely burned, unglazed stoneware tiles have greater surface hardness and wear resistance than glazed tiles but glazed tiles offer a wider range of possibilities for decorative surfaces. This includes not only the use of ceramic decorative finishes on or under the glaze but also a wider range of colors for glazes.
Conventional unglazed ceramic stoneware compositions which burn to a light color and have suitable characteristics for dense burning and the required stain resistance are fired at temperatures above 1200.degree. C. As their burned color is non-white, however, they cannot be obtained in luminous and saturated color shades. Moreover, they are frequently sensitive to stains due to a slight residual porosity. Added to this is the problem that the clays required for producing densely burned stoneware tiles with a light burned color are not always readily available. Only a few natural deposits are known, and these have only limited reserves (e.g. Westerwald).
Stoneware or semi-stoneware tiles fired at low temperatures and having a water absorption capacity greater than about 0.3% by weight are generally sensitive to staining if they have not been glazed. For obtaining closed, dense surfaces, these products must be glazed. On the other hand, clays which are widely available but burn to a dark color may be used for the manufacture of such products.
The problem therefore arose of providing compositions which, when applied as a relatively thick top layer to a basic (more or less colored) stoneware or semi-stoneware body, could be used for the production of stoneware tiles which at least on the surface sinter to a dense structure, are abrasion resistant and can be colored in luminous colors with a high degree of saturation with coloring materials even at relatively low concentrations. Semi-stoneware is generally understood as having a residual porosity of 3-6%.
This problem has been solved by means of the compositions according to the invention.